1. Technical Field
This invention relates to linear beam tubes and more particularly to setting and maintaining the heater voltage of these and other vacuum electron devices (VEDs).
2. Description of Related Art
It is generally known that the higher the heater voltage applied to the cathode of a vacuum electron device, the higher the operating temperature of the cathode. One of the major factors limiting cathode life in a klystron tube, for example, is an excessive cathode operating temperature for a given cathode type. The majority of users of klystron tubes are accustomed to a constant heater voltage as the klystron requirements usually specify fixed values.
An example is the semi-manual adjustment method used at the Stanford Linear Accelerator Center (SLAC) in Stanford, Calif. In the SLAC, one of the linear accelerators includes two hundred forty S-band (2856 MHz) klystrons operating at power levels exceeding 60 MW to provide the RF power to the accelerator. Technicians periodically adjust the heater voltage when lower than the normal klystron output power (cathode current) is observed. The life expectancy reported in this mode of operation at a current level of 6 A/cm2 exceeds 50,000 hours in pulsed mode. At 2 A/cm2 cathode loading, the expected life of a klystron should exceed 100,000 hours.
This invention relates to a beam current monitoring system for a vacuum electron device including a power supply for generating the heater beam voltage in the vacuum electron device. A beam current monitor monitors the cathode beam current in the vacuum electron device and indicates to the power supply the heater and beam voltage to the vacuum electron device to generate the beam current. The system includes a microprocessor under software program management for controlling the beam voltage indicator in response to the beam current monitor. The software program includes an algorithm for maintaining the cathode of the vacuum electron device at the lowest temperature required for a predetermined percent of the beam current that corresponds to a fully space charge limited operation of said vacuum electron device.
A method is also disclosed for setting the heater voltage in said vacuum electron device to a value corresponding to the beam current for the fully space charge limited (FSCL) operation of the VED. The heater voltage is lowered to a first predetermined percent of FSCL, then the heater voltage is then increased to generate a higher beam current that corresponds to a second, higher, predetermined percent of the beam current that corresponds to a FSCL operation of said VED.
Also disclosed is a computer readable medium including program instructions for setting and maintaining vacuum electron device heater voltage performing the following: Monitoring the beam current in the vacuum electron device at a predetermined percent of the beam current that corresponds to a fully space charge limited operation of the vacuum electron device. And adjusting the heater voltage causing the beam current at the predetermined percent if and/or when the beam current differs from the predetermined percent.